1. Field of the Invention
The present invention is related to communication systems and more specifically to reducing interference while communicating.
2. Description of the Prior Art
In any electronic circuit there is a certain amount of interference which must be controlled. This interference can be any electrical or electromagnetic disturbance, phenomena, signal, or emission, man-made or natural, which causes or can cause an undesired response, malfunctioning, or degradation of the electrical performance of electrical equipment. To reduce or eliminate this interference there are several prior art techniques. One such technique is discussed in Adaptive Signal Processing, Widrow, B. and Stearns, S., Prentice-Hall, Inc., Englewood Cliffs, N.J. on pages 349 and 350. Here, examples are given where a broadband signal is corrupted by periodic interference and no external reference input, free of the original signal, is available. These examples include the playback of speech or music in the presence of tape hum or turntable rumble or sensing seismic signals in the presence of vehicle engine or powerline noise.
As described in this prior art publication, periodic interference is cancelled by inserting a fixed delay into a reference input which is drawn directly from the primary input. This fixed delay may be inserted in the primary instead of the reference input if its total length is greater than the total delay of the adaptive filter. Otherwise, the adaptive filter will converge to match it and cancel both signal and interference. The fixed delay chosen must be of sufficient length to cause the broadband signal components in the reference input to become decorrelated from those in the primary input. The interference components, because of their periodic nature, will remain correlated with each other.
The deficiencies with the above approach and other similar approaches of the prior art relate to the size of the delay inserted in the reference input or primary input and the overall performance of the interference cancellor circuit.